1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a magneto-optical medium recording and/or reproducing apparatus capable of recording and/or reproducing both from a magneto-optical medium in which a reproduced signal is read out from only one portion of a light radiated area (laser beam spot) upon reproducing to thereby record and/or reproduce an information at high density and from a magneto-optical recording medium of the conventional system in which a reproduced signal is read out from substantially the whole area of the laser beam spot.
2. Description of the Prior Art
An erasable magneto-optical disc has a magneto-optical recording layer. When this magneto-optical layer is irradiated with a laser beam and then heated, the magnetization direction (recording pit) of the heated portion is converted into a magnetization direction corresponding to the external magnetic field associated with a recording information. In this way, an information signal can be recorded. Upon playback, the recorded information signal is played back by utilizing a Kerr effect in which a laser beam is irradiated on the track of the recording pit and a polarized plane of a reflected light is rotated by the magnetization direction. In the case of multi-layer magneto-optical discs having a reflecting layer in addition to the magneto-optical layer, a Faraday effect also is utilized.
A recording linear density of an information on the magneto-optical disc is determined by a carrier-to-noise (C/N) ratio of a reproduced signal. In the magneto-optical recording and/or reproduction of the conventional magneto-optical disc (hereinafter this conventional magneto-optical disc is referred to as an MO disc), as shown in FIG. 1, substantially the whole area of a beam spot 5, i.e., the laser beam light radiated area on the MO disc is employed as a reproduced signal detection area so that the linear recording density of the MO disc, which can be reproduced, is determined by the spot diameter of the laser beam.
If a diameter d of the laser beam spot 5 is smaller than a pitch .tau. of a recording pit 4 as shown in FIG. 1A, then two recording pits 4 cannot enter the laser beam spot 5 and a reproduced output has a waveform shown in FIG. 1B, thus making it possible to read the reproduced signal. However, as shown in FIG. 1C, if the recording pits 4 are formed at high density and the diameter d of the laser beam spot 5 becomes larger than the pitch .tau. of the recording pit 4, then two recording pits 4, for example, enter the same laser beam spot 5 simultaneously and therefore the waveform of the reproduced output becomes substantially constant as shown in FIG. 1D. As a consequence, the two recording pits 4 cannot be reproduced separately and the reproduction becomes impossible.
The spot diameter d depends upon a wavelength .lambda. of the laser beam and an numerical aperture NA of an objective lens. Accordingly, it has been proposed to make the recording high in density by utilizing a laser light of short wavelength .lambda. or by reducing the spot diameter d of the laser beam by increasing the numerical aperture NA of the objective lens. However, these proposals have unavoidable limits from a laser light source and optical system standpoint and these unavoidable limits hinder the magneto-optical disc from becoming high in recording density.
Similarly, a track density is mainly restricted by a crosstalk component from adjacent tracks. In the prior art, the amount of the crosstalk component depends upon the laser beam spot diameter d, which also hinders the the magneto-optical disc from becoming high in recording density.
The assignee of the present application has previously proposed a novel magneto-optical disc in which a readable linear recording density and the track density can be increased without varying the laser beam spot diameter and a method of reproducing such novel magneto-optical disc (see Japanese Laid-Open Patent Publication No. 3-88156 corresponding to U.S. Pat. No. 5,168,482). This novel magneto-optical disc will hereinafter be referred to as an MSR (magneto-optical super resolution) disc.
In this MSR disc, by effectively utilizing a temperature distribution provided by the relative movement of the magneto-optical recording medium and the reproducing laser spot 5, the recording pits 4 of the magneto-optical recording medium will be read only from a predetermined temperature area upon playback, thereby a resolution and density can be increased.
Two types of MSR disc systems are the rear aperture detection type and the front aperture detection type.
First, the rear aperture detection type MSR disc reproducing system will be described with reference to FIGS. 2A and 2B.
FIG. 2A is a schematic plan view illustrating a recording pattern of a magneto-optical recording medium 10 and FIG. 2B is a schematic cross-sectional view illustrating the magnetization state of the magneto-optical recording medium 10. In this case, as shown in FIG. 2A, the magneto-optical recording medium 10 is moved in the direction shown by an arrow D relative to the laser beam spot 5 formed by the laser beam. As shown in FIG. 2B, for example, the magneto-optical recording medium 10, has three layers including a reproducing layer 11 (formed of at least a vertical magnetization layer), and a recording layer 13 and, more preferably, an intermediate layer 12 interposed between the two layers 11 and 13. In FIG. 2B, solid line arrows schematically indicate the directions of the magnetic moment and in the illustrated example, the downward arrows indicate initial state, e.g., "0" in binary value. Further, in FIG. 2B, the upward arrows, i.e., magnetic domains formed by the upward magnetization indicate "1" of binary value and in this state, an information recording pit 4 is formed at least on the recording layer 13 in the form of "1".
A reproducing mode in such magneto-optical recording medium 10 will be described below.
Initially, by the application of an initialized magnetic field Hi from the outside, the reproducing layer 11 is magnetized in the downward direction in FIG. 2B and is thereby initialized. That is, magnetization of the reproducing layer 11 becomes uniform, i.e. uniformly "0," over pits and non-pit areas. After this initialization of the reproducing layer 11, the magnetization directions of the reproducing layer 11 and the recording layer 13 are held in the opposite direction by magnetic walls produced in the intermediate layer 12 in the area of the recording pits 4, where, the recording layer has a value of "1." These pits in which the recording and reproducing layers exhibit opposing magnetization directions are called latent image recording pits 81.
On the other hand, the magneto-optical recording medium 10 is supplied at least at its reproducing portion with a reproducing magnetic field H.sub.r whose direction is opposite to that of the initialized magnetic field H.sub.i. In this state, when the area having a latent image recording pit 81 comes under the laser beam spot 5, its temperature will increase due to the laser irradiation. Portions of the surface of the medium 10 which are irradiated for a longer time reach a higher temperature. The hatched high temperature area 14, shown in FIG. 2A represents a portion of the surface of the medium 10 which has been so heated by the laser beam spot 5. It will be noted that the high temperature area 14 makes up only a portion of the entire laser beam spot 5. When a latent image recording pit 81 reaches this high temperature area 14, the magnetic wall of the intermediate layer 12 breaks down and the magnetization of the recording layer 13 is transferred to the reproducing layer 11 by an exchange force, whereby the latent image recording pit 81 existing in the recording layer 13 is embossed on the reproducing layer 11 as the reproducible recording pit 6.
Accordingly, if the rotation of the polarizing plane of the laser beam spot 5 by the Kerr effect due to the magnetization direction in the reproducing layer 11 or due to the Faraday effect is detected, then the recording pit 4 can be read out. However, unless a latent image recording pit 81 has reached the high temperature area 14 of the laser beam spot 5, the latent image recording pit 81 is not embossed on the reproducing layer 11. Therefore the reproducible recording pits 6 exist only in the high temperature area 14 of narrow width. As a consequence, even when a plurality of recording pits 4 are entered into the laser beam spot 5, that is, even in the magneto-optical recording medium 10 of high density recording type, only the reproducible recording pits 6 can be read out, which can make it possible to perform the reproduction at high resolution.
In order to carry out the above-mentioned playback of high resolution, the initialized magnetic field H.sub.i, the reproduced magnetic field H.sub.r, coercive force of each magnetic layer, thickness, magnetization, magnetic wall energy or the like are selected in response to temperatures of the high temperature area 14 and of the low temperature area 15 within the laser beam spot 5. More specifically, assuming that H.sub.C1 represents a coercive force of the reproducing layer 11, M.sub.S1 a saturated magnetization thereof and h.sub.1 a film thickness thereof, then a condition for initializing only the reproducing layer 11 is given by the following equation (1): EQU Hi&gt;H.sub.C1 +.rho..sub.W2 /2M.sub.S1 h.sub.1 ( 1)
where .rho..sub.W2 is the magnetic wall energy between the reproducing layer 11 and the recording layer 13.
Further, assuming that H.sub.C3 represents a coercive force of the recording layer 13, M.sub.S3 a saturated magnetization thereof and h.sub.3 a film thickness thereof, then a condition such that the information of the recording layer 13 is maintained by the magnetic field is given by the following equation (2): EQU Hi&lt;H.sub.C3 -.rho..sub.W2 /2M.sub.S3 H.sub.3 ( 2)
In order to maintain the magnetic wall provided by the intermediate layer 12 between the reproducing layer 11 and the recording layer 13 even after the initialized magnetic field Hi, the condition expressed by the following equation (3) must be established: EQU H.sub.C1 &gt;.rho..sub.W2 /2M.sub.S1 H.sub.1 ( 3)
Then, at a temperature T.sub.H selected within the high temperature area 14, the condition expressed by the following equation (4) must be satisfied: EQU H.sub.C1 -.rho..sub.W2 /2M.sub.S1 H.sub.1 &lt;H.sub.r &lt;H.sub.C1 +.rho..sub.W2 /2M.sub.S1 h.sub.1 ( 4)
By the application of a reproducing magnetic field H.sub.r which satisfies the above-mentioned equation (4), the magnetization of the latent image recording pit 81 of the recording layer 13 can be transferred, i.e., embossed on the reproducing layer 11 only at its portion where the magnetic wall provided by the intermediate layer 12 exists.
While the magneto-optical recording medium 10 of the MSR type is composed of the reproducing layer 11, the intermediate layer 12 and the recording layer 13 in a trilayer structure, the magneto-optical recording medium 10 is not limited to the trilayer structure and may be applied to a four-layer structure in which a reproducing auxiliary layer 91 is provided on the intermediate layer 12 side of the reproducing layer 11 as shown in a schematic enlarged cross-sectional view forming FIG. 3.
The reproducing auxiliary layer 91 assists the characteristics of the reproducing layer 11. By this reproducing auxiliary layer 91, the coercive force of the reproducing layer 11 can be compensated for at room temperature, and the magnetization of the reproducing layer 11 arranged by the initialized magnetic field H.sub.i can stably exist regardless of the existence of the magnetic wall. Further, the coercive force rapidly decreases near a reproducing temperature so that the magnetic wall confined within the intermediate layer 12 will spread to the reproducing auxiliary layer 91. Also, the magnetic wall will still break down satisfactorily, even with the auxiliary layer 91, and thereby the recording pit 4 can be embossed satisfactorily.
When the magneto-optical recording medium 10 is formed in a four-layer structure fashion in which the reproducing auxiliary layer 91 is provided as described above, the coercive force H.sub.C1 of the reproducing layer 11 is replaced with a coercive force H.sub.CA given by the following equation (5) and .rho..sub.W2 /M.sub.S1 h.sub.1 is replaced with .rho..sub.W2 /(M.sub.S1 h.sub.1 +M.sub.SS h.sub.S): EQU H.sub.CA =(M.sub.S1 h.sub.1 H.sub.C1 +M.sub.SS h.sub.S H.sub.CS)/(M.sub.S1 h.sub.1 +M.sub.SS h.sub.S) (5)
(inequality of H.sub.C1 &lt;H.sub.CA &lt;H.sub.CS is established in the above-mentioned rear aperture detection type MSR disc) where M.sub.SS, h.sub.S and H.sub.CS represent the saturated magnetization, the film thickness and the coercive force of the reproducing auxiliary layer 91, respectively.
The MSR disc of the front aperture detection type will be described next with reference to FIGS. 4A and 4B. FIG. 4A is a schematic top view illustrative of the recording pattern of the magneto-optical recording medium 10 and FIG. 4B is a schematic cross-sectional view illustrative of the magnetization state. In FIGS. 4A and 4B, like parts corresponding to those of FIGS. 2A and 2B are marked with the same reference numerals and therefore need not be described in detail. In this case, the initialized magnetic field H.sub.i is not required.
The reproducing mode of such magneto-optical recording medium 10 will be described. In this case, the following equation (6) must be established in the high temperature area 14 so that, even within the laser beam spot 5, the magnetizations of the reproducing layer 11 which reach the high temperature area 14 are converted to the downward direction in FIG. 4B by the reproducing magnetic field H.sub.r applied from the outside, thereby the recording pit 4 in the reproducing layer 11 is no longer reproducible. That is, in this MSR disc of the front aperture detection type, the resolution can be increased by reproducing recording pits 4 only within the low temperature area 15 of the beam spot 5. EQU H.sub.r &gt;H.sub.C1 +.rho..sub.W2 /2M.sub.S1 h.sub.1 ( 6)
At that time, under the condition that the recording pit 4 is unreproducible, various conditions such as a coercive force or the like are set in such a fashion that the recording pit 4 is left as a latent image recording pit 81 in the recording layer 13. Thus, at room temperature, the magnetization of the recording layer 13, i.e., the recording pit 4 will be transferred to the reproducing layer 11 and returns to the reproducible condition.
According to the above-mentioned MSR discs of the rear aperture detection type and the front aperture detection type, since the recording pit in the area of one portion of the reproducing laser beam spot is reproduced, the resolution in the playback mode can be improved.
Further, it has been proposed that a magneto-optical recording medium be made in which the above-mentioned two MSR discs of the rear aperture detection type and the front aperture detection type are combined and the zones of varying temperature within a laser beam spot are utilized to further increase density and resolution. Specifically, the area of a magneto-optical recording medium under the laser beam spot 5 will have a high temperature area 14, an intermediate temperature area 16 and a low temperature area 15 (shown in FIG. 5). This allows the high temperature area 14 to function as the MSR disc of the front aperture detection type described in FIG. 4 and also to thereby allow the intermediate temperature area 16 and the low temperature area 15 to function as the two temperature areas necessary for a rear aperture MSR as described in connection with FIG. 2.
According to the MSR disc provided by the combination of the rear aperture detection type MSR disc and the front aperture detection type MSR disc, since the reproducible recording pit 19 as shown by the hatched area in FIG. 5 is limited in the narrow intermediate temperature area 16 sandwiched between the high temperature area 14 and the low temperature area 15, the resolution in the playback mode can be improved more.
Incidentally, it is preferable that the MSR disc suitable for recording and reproducing of high resolution can be recorded and/or reproduced by an ordinary magneto-optical disc drive apparatus according to a recording and/or reproducing system which will be described below with reference to FIGS. 6 and 7.
That is, for disc medium used in the data storage such as external storage of a computer, in order to facilitate the data processing and the data access, the track area on a disc medium D is divided at every sector S of a proper length so that data can be processed in units defined by the sector S as shown in FIG. 6. Then, sector control information such as a physical address on the disc D or the like are recorded on each sector S and the sector control information is written in advance in the disc as an emboss signal.
FIG. 7 shows an ISO standard sector format of the WO (write once optical disc)/MO (erasable type optical disc). As shown in FIG. 7, one sector is composed of a header portion HD and a recording data portion DA, and the header portion HD is recorded (pre-formatted) on the optical disc medium in advance as the emboss signal as earlier noted. The header portion HD is composed of a sector synchronizing (sync.) portion and an address portion. The sector sync. portion is used to relatively identify the interval between the sectors and sector control information such as physical address on the disc or the like are recorded on the address portion. The physical address is composed of, for example, a track address and a sector address. In some cases, physical addresses might be sectors having serial numbers. The recording data is recorded only in the recording data portion DA in association with the sector control information of the header portion HD (associated information is stored in a directory area).
However, since the magneto-optical recording medium, particularly, the magneto-optical recording medium of the front aperture detection type is arranged so as to read the recorded signal by changing the magnetization state of the reproducing layer in the playback mode, the magnetic characteristic thereof is changed at a relatively low temperature in such a manner that the magnetization is changed under a predetermined temperature condition. Accordingly, if the above magneto-optical recording medium is recorded and/or reproduced by the ordinary magneto-optical disc drive apparatus, the magnetic characteristic becomes unstable and a reproduced output fluctuates. There is then the risk that the recorded signal cannot be played back precisely.
Furthermore, considering the recording and/or reproducing apparatus of the MSR disc, it is preferable that the MO disc, which is now widely and commercially available on the market, can be recorded and/or reproduced by this recording and/or reproducing apparatus. In that case, it is preferable that the recording and/or reproducing apparatus can use common hardware for recording and/or reproducing the above two discs, thereby simplifying the arrangement.